Conventional polyethylene terephthalate ("PET") resins generally have poor melt strength. Hence, molten PET tends to quickly collapse when foamed. Conventionally foamed PET can also have poor mechanical properties, due to broad differences in cells sizes, cell wall thicknesses and the like. Furthermore, conventional chlorinated fluorocarbon foaming agents are problematic, since they are deemed harmful to the environment. New foaming agents have been developed, but can be problematic for a variety of reasons.
Hence, although a large potential market exists for foamed PET, cost, environmental concerns and manufacturing complexities have hindered the production of such materials.